OBD-II Code · Powertrain
P0303
Cylinder 3 Misfire Detected
Severity
high
Left alone, this one tends to take other parts with it.
Can you drive it
Depends on the light
Flashing: stop as soon as it is safe to — raw fuel is reaching the catalytic converter. Steady, and it only feels rough: a short drive to a shop is usually fine. Either way, stop if it shakes hard, loses power, overheats or makes a new noise.
Indicative cost (USD)
$30–$400
Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
Swap the coil to a quiet cylinder before buying parts
Cylinder 3 misfire.
How a shop works this
The ECU has already told you which cylinder. That makes the fastest honest test a swap: move one part to another cylinder and see whether the fault moves with it. It identifies the fault instead of guessing at it, and it costs nothing.
Photograph the coils before you touch anything, and note which is which.
Cylinder numbering is not the same across manufacturers, and V6 and V8 bank layouts differ by maker — look up the firing order and bank layout for your engine before you move parts, or you can spend an hour swapping coils on the wrong bank.
Move the cylinder 3 ignition coil to a cylinder that is not misfiring, put that cylinder's coil on 3, clear the code, and run it until the misfire returns.
On a modern coil-on-plug engine the coil is the most common single answer to a cylinder-specific misfire, which is why it is the first thing to move — not because it is the cheapest part. A misfire that is there even at idle is the classic dead-coil pattern.
- The misfire follows the coil to its new cylinderThe coil is the fault. Replace it. If the engine has real mileage on it, the other coils are on the same clock.
- The misfire stays on cylinder 3The coil is fine. Look at the coil connector and the harness around it next — chafed and rodent-damaged connections at that plug are a common second answer — then pull the plug.
Read the plug you pulled, and check the boot for carbon tracking.
Black and sooty means it has been running rich. White or blistered means lean or too hot. Oil-wet points at valve seals or rings. Fuel-wet points at an injector holding open. A carbon-tracked boot arcs to the head and imitates a dead coil for the price of a new boot. On an older engine with a distributor, the cap, rotor and plug wires belong in this step too.
If spark is accounted for, go to fuelling, then to compression.
An injector balance or flow test comes first because it is quicker. Compression test second: a cylinder more than about 15% below its neighbours is mechanical, and no ignition part will fix it. A vacuum leak on this one runner sits alongside these — a leak big enough to affect every cylinder would be setting P0300 instead.
A flashing check-engine light means the misfire is putting raw fuel into the catalytic converter, which can destroy it quickly. Stop driving as soon as it is safe to. A steady light with a mild misfire is less urgent, but stop anyway if it shakes hard, loses power, overheats, or makes a noise it was not making before.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Rough idle
- Engine stumble
- Check engine light
- Vibration under load
Likely causes
- Failed ignition coil (cylinder 3)Most common
- Damaged coil connector or harness at cylinder 3
- Worn spark plug or carbon-tracked boot (cylinder 3)
- Clogged or leaking fuel injector (cylinder 3)
- Low compression on cylinder 3
- Vacuum leak at the cylinder 3 intake runner
Read the full diagnostic procedureP0303 means cylinder 3 is misfiring.
P0303 means cylinder 3 is misfiring.
Same shop-floor diagnostic flow as P0301/P0302
- pull the plug from cylinder 3, read it, swap the coil with an adjacent cylinder, clear the code, and confirm whether the misfire follows the coil to the new cylinder.
- Critical caveat for V6/V8 work: cylinder numbering is NOT the same across manufacturers, so you have to look up the firing order and bank layout before you start swapping.
- On a transverse Honda/Toyota V6, cylinders 1-3 are typically on the rear bank (closest to the firewall) and 4-6 on the front; on a Chrysler 3.6L Pentastar, 1-3-5 are on the right bank and 2-4-6 on the left; on a Ford 5.4L 3-valve V8, the right bank is 1-2-3-4 and the left bank is 5-6-7-8, which is the opposite of GM's LS-series layout.
- Grab the correct cylinder before you waste an hour swapping coils on the wrong bank.
Vehicle-specific patterns2008-2013 Honda Accord/Odyssey/Pilot with the J35 V6 frequently throws P0303 from the Variable Cylinder Management (VCM) system failing to fully reac…
- 2008-2013Honda Accord/Odyssey/Pilot with the J35 V6 frequently throws P0303 from the Variable Cylinder Management (VCM) system failing to fully reactivate cylinder 3 after deactivation cycles; VCMTuner-style disable modules are a common shop fix but a band-aid — the real fix is the VCM solenoid or a software update.
- 2007-2010BMW N54 (335i, 535i) throws P0303 from the well-documented HPFP (high-pressure fuel pump) issue and from injector index-code mismatch when injectors are swapped without recoding.
- 2005-2010Ford Mustang/F-150 with the 4.6L 3V V8 commonly throws P0303 from coil-boot carbon tracking — the COP coils sit in deep wells that hold heat and oil mist.
Estimated repair$40 (single coil) to $1,400+ (HPFP replacement on the N54).
Look up another code
P0303 — frequently asked
What does a P0303 code mean?
Cylinder 3 misfire.
What causes a P0303 code?
Common causes: Failed ignition coil (cylinder 3); Damaged coil connector or harness at cylinder 3; Worn spark plug or carbon-tracked boot (cylinder 3); Clogged or leaking fuel injector (cylinder 3); Low compression on cylinder 3; Vacuum leak at the cylinder 3 intake runner.
What are the symptoms of a P0303 code?
Typical symptoms: Rough idle; Engine stumble; Check engine light; Vibration under load.
Is it safe to drive with a P0303 code?
Depends on the light. Flashing: stop as soon as it is safe to — raw fuel is reaching the catalytic converter. Steady, and it only feels rough: a short drive to a shop is usually fine. Either way, stop if it shakes hard, loses power, overheats or makes a new noise. Estimated repair: $30-$400 — swap the coil to a quiet cylinder before buying parts.
How do I diagnose a P0303 code?
The ECU has already told you which cylinder. That makes the fastest honest test a swap: move one part to another cylinder and see whether the fault moves with it. It identifies the fault instead of guessing at it, and it costs nothing. 1. Photograph the coils before you touch anything, and note which is which. 2. Move the cylinder 3 ignition coil to a cylinder that is not misfiring, put that cylinder's coil on 3, clear the code, and run it until the misfire returns. 3. Read the plug you pulled, and check the boot for carbon tracking. 4. If spark is accounted for, go to fuelling, then to compression.
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